Digital modulation schemes, such as .pi./4 differential quadrature phase shift keying (.pi./4 DQPSK), are used in transmitters for North American Digital Communications (NADC) and Personal Digital Communications (PDC) wireless communication systems. Alternative digital modulation techniques used in other wireless communication systems include other variations of quadrature-amplitude modulation (QAM) such as 64-QAM, 16-QAM, and QPSK. A conventional .pi./4 DQPSK modulator includes an encoder, finite-duration impulse response (FIR) pulse shaping filters for in-phase and quadrature (I/Q) signals, and a quadrature modulator.
Both NADC and PDC use square root raised cosine (SRRC) FIR pulse shaping filters to limit the bandwidth of the transmitted signal and limit interference with adjacent frequency channels. Unfortunately, these FIR pulse shaping filters also impart amplitude modulation onto the transmitted waveform. This amplitude modulation may be characterized by a peak-to-average power ratio. Higher peak-to-average power ratios result in decreased radio frequency (RF) power amplifier efficiency, because the RF power amplifier in the transmitter must remain substantially linear over the full range of the signal envelope. In a portable device, the RF power amplifier is a major source of power consumption, and reduced efficiency has a significant impact on battery life. Additionally, the cost of a linear amplifier increases as the peak power of the transmitted signal increases.
Thus, there is a need for a digital modulator that produces a modulated signal with a reduced peak-to-average power ratio to decrease both power consumption and cost in a communication system transmitter.